What a wonderful thing you are doing. I've heard of these little dresses in the past & think it's a wonderful thing to make & send to the girls.
You might try contacting manufactures of bias tape & see if they are willing to donate some for your cause. I've found that sometimes companies like that have a certain amount that they will donate to each year. The worst thing that could happen would be that they say no....the best is that they send you tons of it. You might also tell them that any color can be used & that tape that didn't meet all the quality controls or discontinued colors would be acceptable also. If you need a specific width, be sure to mention that in the request.
I've been known to also include a link to the organization that I am helping & a contact name & number of someone who can verify that my request is legitimate so they don't think I'm using it for myself.
You could do the same with trim & lace & button manufacturers.